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Full text of "Handbook Of Chemical Engineering - I"

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MATERIAL HANDLING—TRANSPORTATION OF SOLIDS         83
By formula (3):
C = (20Q X 0.003 + (1 + 1-2) +25 X 0.0009) 381 + 0.165 X 4000 X 25 0                                                       80,000
= $0.0341—Net operating cost per ton. Result.—Net operating cost, 3.41 cts. per ton, duration of task, 48 8-hr, working days.
Hoists and Cranes.—Under this head may conveniently be grouped: (1) Simple mast and gaff rigs used for loading and unloading loose material by means of buckets, scoops, magnets, etc., which have a conveying radius controlled by the length of the gaff or boom of the rig. (2) Hoisting towers which are used primarily for unloading purposes, hoisting the material and transporting it to a greater or lesser extent according to whether the tower is of the traveling or stationary variety. (3) Overhead cranes which raise and lower loads, distributing them by means of trolley travel on the bridge and the movement of the bridge on its supporting rails. (4) Miscellaneous cranes such as those of the wall and jib types which have a more restricted area of operation and those special cranes developed for particular purposes.
Economic comparisons and detailed investigations into the cost of operating all these various types of hoists and cranes cannot well be made here, but an analytical consideration of the more complicated type, the overhead cranes, will serve to demonstrate approved methods for arriving at such data.
Overhead Cranes.—These are usually electrically operated, consisting of a hoist mounted on a traveling trolley running on a transverse, overhead bridge which in turn travels on supporting rails or tracks. The hoists employ buckets, tubs, magnets, slings, etc. for holding the material handled.
Overhead electric cranes are ordinarily rated as of 5-, 10-, 15-, 20-, 25-, 30-, 40-, 50-, 60-, 75-, 100- or 150-ton lifting capacity at its minimum, or first, hoisting speed. At higher hoisting speeds, its lifting capacity is reduced proportionally. Cranes are classified according to their hoisting speeds and are customarily classed as low-, medium- or high-speed. Each class of crane is provided with three hoisting speeds —the crane developing its full rated lifting capacity at first hoisting speed. At other than first speed, the lifting capacity is:
~       K X first speed                              „         ,    /A.
Q = r\-----TV—^ -j                              Formula (4)
Operating speed
Where, K = rated capacity of crane in tons at first speed, and
Q = Lifting capacity of crane in tons at operating speed.
Hoisting speeds of overhead cranes are not standardized, but practice has fairly established speed ratios approximately as given in Table 5. Trolley speeds vary from 100 to 150 ft. per minute. Bridge speeds vary from 300 to 500 ft. per minute for small cranes and from 200 to 300 ft. per minute for large cranes.
Power.—Power requirements of overhead cranes for hoisting, trolley and bridge motors depend upon speed of travel (minimum) and the load lifted (maximum capacity). The power of the motors are thus subject to selection to a certain extent and is governed by the Formulas (5o), (56) and (5c):
() v WSf Horsepower of hoisting motor =         „                                      Formula (5a)
Q X TS Horsepower of trolley motor =     Af\r\                                      Formula (56)
Horsepower of bridge motor =----    OOK   —-                       Formula (5c)